1 Field of the Invention
The present invention relates to a pump controller for a construction machine equipped with a hydraulic circuit to execute a negative control.
2 Description of the Related Art
FIG. 10 illustrates a general negative control circuit in a hydraulic excavator. In the negative control circuit shown in the drawing, the discharge port of a variable displacement hydraulic pump 30 is connected to a tank 34 by a center bypass 31 through a plurality of control valves, for example, a swing motor control valve 32 and an arm control valve 33. Each of the control valves 32, 33 is supplied in parallel with a pressure oil discharged from the variable displacement hydraulic pump 30 in the operation mode.
A negative control restrictor 35 is installed on the downstream of the arm control valve 33 in the center bypass 31. An upstream point a to the negative control restrictor 35 is connected by a hydraulic signal line b to a regulator 36 that varies a discharge from the hydraulic pump 30. Thereby, the negative control is designed such that when the pressure of the hydraulic signal line b becomes low, the discharge from the hydraulic pump 30 is increased, and when the pressure of the hydraulic signal line b becomes high, the discharge is decreased. Here in the drawing, the symbol 37 and 38 signify a swing motor and an arm cylinder, as actuators, each supplied with a pressure oil in which the flow rate and direction are controlled.
In the circuit, the discharge from the variable displacement hydraulic pump 30 is determined by the magnitudes of the loads to the actuators and the sizes of the bleeding-off control restrictors of the control valves 32, 33. However, since the magnitudes of the loads vary from moment to moment, the discharge in the hydraulic circuit is set appropriately although the minimum and maximum discharges are grasped.
However, in a work with a hydraulic excavator, there is such a situation that the maximum speed in the swing mode is desirably suppressed, and the speed of the arm is desirably not to be lowered.
As the measures for this case, as shown in the opening area characteristic (in FIG. 11) of the control valves 32, 33, the maximum speed of a specific actuator can be lowered by making the opening area characteristic of the bleeding-off control restrictor without a closure. However, when the pressure on the load is high, since the flow rate passing through the bleeding-off control restrictor of the control valve increases, the flow rate passing through the negative control restrictor 35 increases, as shown in FIG. 12. As a result, the effect by the negative control extremely lowers the pump discharge, creates a state in which the operation lever has to be manipulated deeper than needed for activating the actuator, or makes the pump discharge insufficient so that the maximum pressure cannot be generated, which is a shortcoming.
And, as shown in FIG. 13, the negative control characteristic is forcibly made into a head suppression, and conceivably the maximum pump discharge is varied for each actuator. However in this case, when an operator manipulates the operation lever of the swing motor 37 in full to switch the swing motor control valve 32 from a position c to a position d (see FIG. 10), since the center bypass 31 is blocked by the swing motor control valve 32, the pressure oil passing through the negative control restrictor 35 runs short, and the pressure of the signal line b becomes low, whereby the discharge from the variable displacement hydraulic pump 30 is increased. Therefore, when the operator manipulates the arm operation lever still further, the pump discharge flow rate increases at a stroke. On the contrary, when returning to a single operation, the pump flow rate rapidly decreases. Thus, since the discharge fluctuates greatly at each time when the actuator is switched from the single operation into the compound operation, or on the contrary, when switched from the compound operation into the single operation, the operator gets into a state of difficulty with operation.
Further, in case a plurality of pumps are controlled independently, the pump maximum discharge is needed to be varied as to a confluent flow. Furthermore, in case a heavyweight optional equipment is mounted on in replacement of a normal front attachment, the pump maximum discharge is needed to be varied in correspondence with the optional equipment.